Conventionally, a seat belt device has been known in which, in order to prevent an occupant from traveling forwardly and colliding with parts of a vehicle or the like at the time of sudden stopping of the vehicle or the like, thereby protecting the occupant, an electromotive retractor serving as a tension varying means capable of allowing a tension given to a seat belt to vary according to a collision state is provided.
For example, Patent Document 1 (JP-A No. 2000-52925 (FIG. 1, Paragraphs 0040, 0041, 0048, and 0049)) discloses a technology relating to a vehicle occupant constraining and protecting device (seat belt device) having an electromotive retractor (seat belt winding device) that winds and unwinds a seat belt for protecting an occupant. In the technology disclosed in Patent Document 1, in order to drive a seat belt winding device, a seat belt winding force is changed according to a tread power on a brake pedal provided in the vehicle and a tension of the seat belt is also changed.
Further, Patent Document 2 (JP-A No. 2001-163185 (FIG. 4, FIG. 6, Paragraphs 0028, and 0041 to 0043)) discloses a technology relating to a seat belt device having a seat belt winding device that includes a collision prediction part for avoiding a collision with an obstacle, such as another vehicle in front of a vehicle. In the technology disclosed in Patent Document 2, a distance between the vehicle and the obstacle, for example, is measured by a non-contact distance sensor, such as a laser radar, and, when the collision is likely to occur, a collision prediction signal is outputted to drive a motor, such that the seat belt winding device winds up the seat belt.
Further, a seat belt device has also been known in which a tension detecting means for measuring a tension given to the seat belt is provided to confirm an attachment state of the seat belt.
For example, Patent Document 3 (JP-A No. 2002-19581 (FIG. 9 and Paragraph 0009)) discloses a seat belt tension sensor having the following configuration. When the tension is given to a seat belt, the seat belt wound around a seat belt winding part pulls the seat belt winding part with respect to a supporting part. By doing so, a pivoting part rotates or is going to rotate around a rotational center. Accordingly, a power transmission part provided at an end of the pivoting part also rotates or is going to rotate, and the one end presses a load measuring part so that a force is exerted on the load measuring part. The force exerted on the load measuring part has a constant relationship with the tension given to the seat belt, so that the tension of the seat belt can be detected by detecting the force.
As regards the seat belt device including the electromotive retractor serving as the tension varying means described above, a seat belt device that has a failure diagnosis system of the electromotive retractor itself incorporated therein has been known.
For example, in Patent Document 4 (JP-A No. 11-170966 (FIG. 5 and Paragraph 0005)), a seat belt device has been suggested in which a failure diagnosing means for diagnosing a failure of a driving means based on a waveform of a current flowing in the driving means, when a predetermined voltage waveform is applied to the driving means for driving an electromotive retractor, is provided.
However, the failure diagnosing means disclosed in Patent Document 4 can detect the failure of the electromotive retractor of the tension varying means, but it cannot diagnose whether or not the tension varying means gives a predetermined tension to the seat belt.
Further, in the seat belt device having the tension detecting means disclosed in Patent Document 3, the failure of the tension detecting means cannot be diagnosed.